Digital systems often include raster scanned display devices, such as cathode ray tube (CRT) monitors, for the display of information. In a typical raster scanned display device, a beam is scanned over a screen or other area in a series of horizontal scan lines. Information is generally displayed on the screen in a character-by-character format, where each character may be a number, an English or foreign letter, or a symbol such as a punctuation or graphics symbol. The screen is divided into a number of character areas, each character area being used to display one character. The character areas are arranged in horizontal rows, and each character area comprises dots or pixels that can be individually and selectively illuminated by the display device so as to form a particular character.
A digital system that includes a raster scanned display device typically controls the display device in part by providing a dot clock signal that has a period corresponding to the time required to horizontally scan one dot. Since the time available to scan and retrace one horizontal scan line is generally fixed, the frequency of the dot clock signal determines the horizontal dot spacing, and the horizontal dot spacing together with the dot width of the character areas determines the number of characters that may be displayed per row. In the past, if a system needed the capability of changing horizontal formats during operation, it was necessary to provide a plurality of dot clock signal generators and means for switching between them. The plurality of signal generators resulted in a greater number of parts, such as oscillators, and therefore added to the expense of the system. The additional oscillators also resulted in increased EMI and RFI problems.